A number of process variables are associated with the operation of a paper making machine headbox, the control of which directly affect the quality of paper produced. For example, at the wet end the slice jet velocity and total head will greatly affect such things as formation and tensile ratio as well as interply bonding.
A significant factor in controlling these properties is believed to be the ideal distance (I.D.) or eddy decay length. In a so called "bunched tube headbox" in which the stock supply chamber communicates with the slice through a plurality of rows of tubes, the ideal distance or eddy decay length, for the eddies created by the changes in velocity as the stock is discharged from the multiple tubes into the slice chamber immediately preceding the slice outlet, may be calculated by the following formula based on a formula by Jasper Mardon: EQU I.D. = K .times. V.sub.AV .times. (d.sub.H).sup.1/2 /(d.sub.R).sup.1/3
wherein K is a coefficient, V.sub.AV is the average velocity through the flow channel interconnecting the stock supply chamber and the slice at 100% open area in feet per second, d.sub.H is the inside diameter of the tubes in the flow channel in inches, and d.sub.R is the depth of the flow passage in a direction normal to the cross machine direction and the length of the tubes.
From the above it will be seen that the ideal distance is directly proportional to V.sub.AV and d.sub.H and inversely proportional to d.sub.R. Thus, the ideal distance or eddy decay length can be increased by increasing the velocity through the flow channel or increasing the diameter of the tubes or by decreasing the effective depth of the flow channel. However, with the head of the stock delivered to the slice being fixed by other considerations and the length and diameter of the tubes in the flow channel also fixed, it would appear that, as a practical matter, the ideal distance or eddy decay length would be a fixed character of a particular headbox. This is the case in almost all current designs.